1. Field of the Invention
This invention relates to tamper indicating closures, and, more particularly, this invention relates to a method of forming plastic closures having a tamper indicating band depending from a cap skirt.
2. State of the Art
There is a wide variety of tamper indicating closures that are used in conjunction with containers to enable the purchaser of the product to determine whether the container has been opened. One of the most popular types of closures includes an internally threaded cap having a tamper indicating band attached to the lower end of the cap skirt by a frangible connection. The band has an inwardly projecting stop which cooperates with an outwardly projecting stop on the container neck to resist relative movement between the band and the container neck causing fracture of the frangible connection when the closure is initially unthreaded from the container neck. The band stop can be in the form of a continuous or segmented inwardly projecting bead which cooperates with an outwardly extending flange on the container neck. Alternatively, the band stop can be in the form of circumferentially spaced, inwardly projecting ratchet teeth which cooperate with circumferentially spaced, outwardly projecting ratchet teeth on the container neck. The frangible connection between the bottom of the cap skirt and the tamper indicating band preferably takes the form of circumferentially spaced, axially extending frangible or breakable webs. The width and thickness of the breakable webs are carefully. selected to assure fracture when normal unthreading torque is applied to the cap but to withstand damage when the cap is initially applied to the container neck.
Normally, the closure is molded with the frangible straps spaced apart by open window areas between the bottom of the cap and the top of the tamper indicating band. U.S. Pat. Nos. 4,572,387, 4,343,408, 4,479,586 and 3,929,246 are examples of closures formed by this molding process. It is difficult to injection mold the straps with a small enough cross section to break properly during removal of the cap. Even where the dimensions of the cross section are precisely controlled, the molecular structure of the plastic is affected by the restricted flow from the cap skirt through the straps to the tamper indicating band, and this change in structure can be evidenced by non-uniform breaking characteristics immediately after molding or after an aging period in the unused closure state or after normal application to a filled container.
In some cases the cap and tamper indicating band are molded as an axially continuous annular wall with no gap delineating the cap skirt from the band. The gap is created later by cutting or slitting in such a way so as to leave the frangible straps.
One approach to the slitting process utilizes a notched cutter which cuts through a uniform closure skirt wall creating the straps and separating windows between the cap and tamper indicating band. This process is illustrated in U.S. Pat. Nos. 4,322,009 and 3,673,761. Obviously, cutter sharpening and cutter life are significant factors when this slitting process is used.
Another approach to the slitting process is cutting to a uniform depth through a non-uniform or ribbed wall thickness leaving internal ribs intact which act as the frangible straps. This process is illustrated in U.S. Pat. Nos. 4,418,828, 4,545,496 and 4,511,054. This also requires frequent cutter sharpening and replacement.